Crystal growing apparatus



Aug. 9, 1966 R. N. HALL CRYSTAL GROWING APPARATUS Filed Sept. 21, 1964 inver-,130m- R b r't' lV. Hal/, by UL'- is Attorney.

United States Patent Ofiice 3,265,469 Patented August 9, 1966 3,265,469 CRYSTAL GROWING APPARATUS Robert N. Hall, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Sept. 21, 1964, Ser. No. 397,704 3 Claims. (Cl. 23-273) The present application is a continuation-in-part of my copending application entitled Crystal Growing Method, Serial No. 60,898, filed October 6, 1960. l

This invention relates to apparatus for growing single crystals of fusible materials such as germanium and silicon and in particular to apparatus for rapidly growing single crystals of small uniform diameter.

In the manufacture of many semiconductor devices such as rectiers and transistors, for example, extremely pure semiconductive material is required. In addition, it is often desirable to form a single crystal of semiconductive material having uniform conductivity of either N-type or P-type throughout its length. The prior art shows many methods of producing single crystals of various materials, a great number of which involve melting and recrystallizing the material. For example, such single crystals have been prepared by drawing a crystal from a mass of melted material.

In one such method of producing single crystals, often called the VCzochralski pulling technique, a seed crystal is dipped into the material which has been melted and maintained at a temperature just barely abovey its melting point after which the seed crystal is slowly withdrawn. As the seed crystal is withdrawn a crystal forms by continuing accretion to the seed. Further details of this method and apparatus suitable for use therewith may be had by reference to the volume entitled Introduction to Semiconductors by W. Crawford Dunlap, Ir., published in 1957 by John Wiley and Sons, Inc., NewYork.

In all the prior art methods of this type the temperature of the melt must be maintained within very close limits. For example, if the temperature of the melt is too high the seed crystal will melt while if too low there will be rapid growth of the crystal ,down into the melt or the melt itself may freeze. The temperature of the melt is usually maintained within the prescribed limits by very accurate control of the furnace power, usually to within 1 C. 0r less. In addition, in order to avoid irregularities in the crystal it must ordinarily be withdrawn from the melt at a slow rate, usually about 3 to 6 inches per hour or less.

While the above crystal growing methods have the advantage that strains and distortion are minimized in the resulting crystal, growth is often slow and the crystal is of a relatively large diameter. The large diameter crystal must be reduced into thin wafers which are then cut into dice for use inthe manufacture of semiconductor devices. Since this often requires several different cutting operations with attendant waste, it is highly desirable to reduce these cutting operations by obtaining crystals of a small uniform diameter.

It is an object of this invention, therefore, to provide apparatus for growing single crystals from a melt which substantially avoids one or more of the prior art disadvantages, l

It is another object of this invention to provide apparatus for producing single crystals of small uniform diameter.

It is another object of this invention to provide apparatus for growing crystals having a small uniform diameter which requires less critical control of furnace power than prior art apparatus.

It is still another objectof this invention to provide apparatus for producing uniform single crystals of small diameter at a more rapid rate than known heretofore in the prior art.

It is yet another object of this invention to reduce waste in the preparation of semiconductor wafers by providing dislocation-free single crystals of semiconductive material having small and uniform diameter.

Briefly stated, in accord with one aspect of this invention, apparatus is provided that permits growing crystals from a melt by maintaining the body of the melt at a temperature above its freezing temperature and maintaining a portion of its surface in a generally convex configuration. The convex configuration is caused by a depending portion of the reaction vessel cover that extends below the natural surface level of the melt and contains a central aperture. A high thermal gradient is maintained in the region of the convexed surface portion without deleteriousicooling of the remaining surface of the melt. This is accomplished by a cooling shield that overlies, but is spaced from, the cover and contains a cylindrical aperture which is in closely spaced coaxial relationship to the central aperture of the cover.

Features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connectionwith the` accompanying drawing which is a diagrammatic view in vertical section of an apparatus suitable for use in the practice of this invention, for growing germanium.

In the drawing there is shown a reaction vessel 1 conveniently fabricated of inert refractory material, as graphite or quartz. Vessel 1 comprises a solid bottom supporting portion 2 with an integral upwardly extending side wall portion 3 forming an openV top receptacle, -or Crucible, suitable for confining a melt 4 of semiconductive material. Bottom supporting portion 2 of vessel 1 rests upon a frame 5 that is, in turn, fixed, as by ange 6, to the bottom of an outer scalable enclosure 7.

Meansl for introducing semiconductive material into vessel 1 are shown in the form of a hollow tube 8 that provides a communicating passage from within vessel 1, near the top thereof, to the outside of enclosure 7. The semiconductive materials (and desired impurities) are preferably introduced in the form of solid granules, however, liquids and gases are equally advantageously employed in some cases.

Means for heating semiconductive material introduced into vessel 1 to a temperature at least as high as the melting temperature thereof is illustrated as a resistance heating element 9 which is connected to a suitable source of electrical energy by externally accessible conductors 10 and 11. Alternatively, vessel 1 can be heated by a high frequency induction coil or other suitable heating means knownin the art. The Crucible, therefore, should be `formed of a material which is thermally responsive to the heating means nad chemically inert with respect to theV material to be melted. If induction heating is utilized, the Crucible should be non-paramagnetic. High purity graphitel fulfills these requirements and has been successfully used for such purposes.

Thev walls of enclosure 7 are designed to enclose the crys-tal growing apparatus and may be of metal or other suitable material. If high frequency induction heating is used, however, the walls of chamber 7 should be preferably of an insulating material such as' quartz. In addition, chamber 7 is fitted with a closure 12 adapted to be secured substantially gas tight by means of gasket 13 and a plurality of bolts` 14.

Vessel 1 is fitted with a cover 15 havingv a depending central portion 16A adapted to press against the surface of the melted material andy cause the same to bulge upward into a general-ly convex configuration as' shown in the drawing. Top cover 15 is fabricated of inert refractory material which may conveniently be selected to be the same material from which vessel 1 is fabricated. A peripheral portion 17 of cover 15 abuts a complementary shaped portion 18 of side wall 3 near the uppermost extremity of side wall 3.

Cover has an inverted frustoconical-shape central portion 16 depending from peripheral portion 17 and having the innermost part 19 thereof adapted to be submerged below the natural surface level 20 of melt 4. The center-most portion of cover 15 comprises an annular lip 21 projecting upwardly from the innermost part 19 of frustoconical shape central portion 16 and adapted to extend upwardly to a-t least as high as level 20 of melt 4. Annular lip 21 encloses a small diameter central aperture 22 formed therein through which a crystal 23 can be continuously withdrawn from the relatively small convexed part 24 of the top surface of melt 4.

Means are provided to maintain a high thermal gradient in the withdrawn crystal as by passing the withdrawn crystal through a cooled region in the vicinity of the convexed portion of the melt from which the crystal is grown. To this end there is provided cooling shield 25 having an inverted f-rustoconical central portion 26 with an under surface having a configuration generally similar to that of the central portion 16 of cover 15 and having a cylindrical aperture 27 through which the grown crystal is passed. Shield 25 is disposed in close juxtaposition to, but spaced from, cover 11 except along its periphery and is formed of a material having good thermal conductivity, such as silver or the like. The peripheral portion 28 of shield 25 engages a complementary portion of the top surface of cover 15 to provide support for shield 25.

Shield 25 has a continuous passage 29 therein for cooling las .by the circulation of water through inlet and outlet tubes 30 and 31, respectively. To maintain a high thermal gradient in the grown crystal near the crystalliquid interface of at least about 400 C. per centimeter and preferably in the range of about 900 C. to 1200 C. per centimeter,.cylindrical aperture 27, that is in closely spaced coaxial relationship to aperture 22, has an axial length equal to about Itwo or three times the diameter of the crystal to be grown. The thickness of central portion 26 through which the grown crystal is passed, therefore, is dimensioned accordingly. For example, for a single crystal having a uniform diameter of approximately one millimeter, central portion 26 ordinarily has a thickness of about two or three millimeters.

The regionin the vicinity of the convexed or bulged up portion 24 of the melt is continually fiushed with an inert or other nonreactive gas. The gas is introduced into this region through conduit 32 which terminates in an annular portion 33 having a plurality of jets 34 therein. Annular portion 33 may be conveniently disposed within thickened portion 26 of annular shield 25. The gas prevents contamination of the melt and aids in maintaining the high thermal gradient in the grown crystal. Excess gas'is removed'from chamber 7 through conduit 35.

The grown crystal 23, in the form of a small diameter wire, is continuously withdrawn from the melt 4 and through the cooled region defined by cylindrical aperlture 27. The grown crystal may be initiated and withdrawn from the convexed portion 24 of melt 4 by conventional means well-known -to the art, which means of-ten impart rotary as well as longitudinal motion to the crystal. Preferably, the crystal is continuously withdrawn from the melt without rotary motion as by means of rollers 36 and 37 and guide 38.

Observation of the growing crystal is provided by a viewing plate 39 of quartz or other suitable transparent material provided in closure 12. To prevent viewing plate 39 from becoming obscured due to fogging or the like, a stream of gas, as from tube 40, may be directed on its inner surface.

The generally convex portion 24 of the melt 4 is maintained throughout the crystal growing operation by adding new material, as for example, through loading conduit 8, to replenish the melt as fast as the crystal is withdrawn. In this way the melt is held at a level 20 at which the depending portion 16 of annularvcover 15 is in contact with the surface of the melt and presses suiiiciently thereon to form `and maintain the generally convex portion.

In operation, heating element 9 is energized to melt the material in vessel 1 and maintain it at a temperature above its freezing temperature and preferably from about 10 C. to 20 C. above. With t-he melted material at an `appropriate level the depending portion 16 of annular cover 15 presses against the surface of the melt and causes a portion thereof to be formed into a generally convex configuration having a small radius of curvature, preferably from Mz to 1/2 centimeter. It has been found that forming a portion of the melt into this generally convex configuration stabilizes crystal growth therefrom resulting in the growth of a crystal which is less dependent upon furnace power than any other method heretofore known to the art. The fact that the Vapparatus 0f the present invention achieves concentrated cooling and a high thermal gradient localized in the immediate vicinity of that portion of the melt surface from which the crystal is grown, without deleterious cooling of the desirably large surface of the melt generally, is believed to substantially contribute to the greatly improved stability.

The region in the vicinity of the generally convexed lportion of the melt is continually flushed with an inert or nonreacting gas as, for example, forming gas cornprising a combination of 10% by volume of hydrogen and the balance nitrogen. The gas is introduced through conduit 32 and the excess removed from chamber 7 through conduit 35.' At the same time water, 4which may conveniently be at room temperature, is caused to circulate through shield 25 by means of inlet and outlet tubes 30 and 31, cooling the shield 25 and its thickened cen- Itral portion 26. The combination of the fiushing gas and cooled annular shield 15 serves to cool both the region in the immediate vicinity of the convexed portion of the melt and the region defined by the aperture 27 through which the grown crystal is withdrawn. The melt in the convexed portion is lowered to a temperature just barely above its freezing temperature and, thus, to the temperature at which a crystal will grow therefrom. Yet the thermal stability attendant use of a large melt is preserved.

A properly oriented seed crystal, as for example, a length of small diameter single crystal previously grown in accord with this invention, is brought into contact with the top of the convexed portion of the melt initiating crystal growth thereat and continuously withdrawn by action of rollers 36 and 37. At the same time, new material is added to the vessel 1, as for example, through loading tube 8 to replenishthe mel-t as fast as the crystal is withdrawn maintaining the generally convexed portion thereof throughout the crystal growing operation in the manner and by the means hereinbefore described.

Since the rate of crystal growth depends primarily upon the temperature of the melt in the vicinity of the growing crystal and the rate of the heat transfer Ifrom the growing crystal, the apparatus of this invention provides for a very rapid crystal growth and withdrawal rate. For example, the generally convex vportion of the melt provides for stable crystal growth while the cooled region in the vicinity thereof acts to maintain the critical-growing temperature of the melt in the vicinity of the growing crystal and removes the heat due to the freezing material forming the crystal. As the grown crystal passes through the cooled region defined by aperture 27 there is a rapid transfer of heat from the crystal. The combination of these features of the present invention results in the growth of single crystals of uniform small diameter at very rapid rates. The crystal may be withdrawn as fast as recrystallization to the seed results, single crystals of uniform diameter of approximately one millimeter being readily grown at rates of 90 inches per hour and more.

In one specific example of the practice of this invention the apparatus shown in the drawing and described hereinbefore is used. 40 grams of high purity germanium is added to vessel 1 having 11A inch inside diameter through loading conduit 8. Forming gas is continuously int-roduced through conduit 32 and removed from chamber 7 through conduit 35. Water is cirC-ulated to annular shield 15 through inlet and outlet tubes 30 and 31 respectively, cooling shield 15 and its thickened Central lportion 26. `600 watts of 60 cycles alternating current power are supplied to resistance heating element 9 for a period of 15 minutes in order to melt the germanium. Additional germanium is added to bring the melted material to a level at which a convexed portion is formed lby contact with the depending yportion 16 of cover 15. When the germanium becomes molten and at a level sufficient to provide the bulgcd-up or convex Portion, heater power is reduced to 500 watts. A seed crystal consisting of a length of single crystal formed previously with the apparatus of this invention, is passed through rollers 36 and 37, through guide 38 into Contact with the top of the convexed portion of the melted germanium. The seed crystal is observed through viewing plate 39 and when it becomes partially melted and integral with the surface of the melt, the seed is withdrawn at an initial rate of approximately 3 inches per hour. Heater power is then gradually reduced as the rate of withdrawal is increased and a crystal having a diameter of about l millimeter is thereafter continuously withdrawn at the rate of 90 inches per hour. New material is added to the melt as fast as the crystal is withdrawn to maintain the convexe-d portion at the surface of the melt throughout the crystal growing operation. At a 'withdrawal rate of 90 inches per hour a single crystal of ge-rmanium is continuously withdrawn having a uniform diameter of approximately 1 millimeter which may be Cut into convenient lengths during the growing operation.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and Changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of this invention.

What I claim as new and desire -to secure by Letters Patent of the United States is:

1. Apparatus for rapidly 4growing a monocrystalline wire of semiconductive material yfrom a melt `of the same, said apparatus comprising:

(a) a Crucible of inert refractory material comprising a solid bottom supporting portion with an integral upwardly extending side wall portion forming an open top receptacle suitable for confining a melt of semiconductive material;

(b) means for introducing semiconducti-ve material to be melted into s-aid Crucible;

(c) means for heating semiconductive material introduced into said crucible to a temperature at ieast yas high as the melting temperature of the semiconductive material;

(d) a top cover of inert refractory material for said Crucible having a peripheral portion thereof abutting the side wall of said crucible near the uppermost extremity of said side wall, said peripheral portion including an annular upper surface, said cover having `an inverted frustoconical-shape central portion depending from said peripheral portion and having the innermost part thereof adapted to be submerged under the surface of a melt Within said Crucible, the centermost portion of said cover comprising an annular llip projecting upwardly from the innermost part of said rustoconical-shape central portion and adapted to extend to at least the top surface of a melt within said Crucible, and said annular lip deiining a small diameter central aperture Ithrough which a crystal can be withdrawn continuously from a relatively small Convexed part of the top surface of adjacent the top of the annular lip of said cover with out subjecting the major surface of a melt within said Crucible to deleterious cooling;

(f) means for continuous-ly withdrawing a grown crystal from the top surface of a melt within the annular lip of said cover through the apertures in said cover and said shield.

2. The .apparatus of claim 1 including la scalable enclosure surrounding said Crucible, said enclosure including :a top closure portion having an aperture therein for guiding withdrawal motion of said `grown crystal from said crucible, said top closure portion further including a transparent viewing pla-te therein and means for introducing a stream of gas into said enclosure, said lastnamed means being oriented to direct the stream of gas onto said transparent viewing plate.

3. Apparatus for rapidly growing a monocrystalline wire of semiconductive material from a melt of the same, said `apparatus Comprising:

(a) a Crucible of inert refractory material comprising a solid bottom supporting portion with an integral upwardly extending side wall portion forming an open top receptacle suitable for confining a melt of semi- Conductive material;

(b) means for Aintroducing semiconductive material to be melted into said Crucible;

(c) means for heating semiconductive material introduced into said crucible to a temperature at least as high as the melting temperature of the semiconduc- Itive material;

(d) a top cover of inert refractory material for said Crucible havin-g a peripheral portion thereof ,abutting the side wall of said Crucible near the uppermost extremity of said side wall, said peripheral portion including an annular upper surface, said cover having an inverted frustoconicall-shape central portion depending ffrom said peripheral portion and having the innermost part thereof adapted to :be submerged under the surface of a melt within said crucible, the centermost port-ion of said Cover comprising an annular lip projectingl upwardly from the innermost part of said frustoconical-shape central portion and adapted to extend to at least the top surface of a melt within said Crucible, and said annular lip dening a small diameter central aperture through which a crystal can be withdrawn continuously from a relatively smalll Convexed part of the top surface of a melt Within said Crucible;

(e) a cooling shield of high thermal conductivity metal having a peripheral portion thereof engaging said annular upper surface of said cover, said shield having a iirst Continuous passage therein for flow of rst cooling medium and a second continuous passage therein for flow of a second cooling medium, and said shield havin-g an inverted frustoconical-shape cen- 7 g trai portion spaced from said cover and depending tal from the top surface of -a melt Within the annular from the peripheral portion of said shield, the cenlip of said cover through the apertures in said cover 4tral portion of said shield having a cylindrical aperand said shield.

ture therein that is lin closely spaced coaxial relationship to the central aperture of said cover and a 5 References Cited by the Examiner plurality of jets therein, each of said jets connecting UNITED STATES PATENTS said second passage to said cylindrical aperture, 2,631,356 3 1953 Sparks et al 23 301 X whereby a high thermal gradient -is established ad- 2,893,347 7 /1959 Schweichert 23 293 jacent the top of the annular lip of said cover with- 3,124,489 3 /1964 Vogej et aL 23....293

out subjecting the major surface of a melt Within said 10 l Crucible to deleterious cooling; NORMAN YUDKOFF, Primary Examzner.

(f) means for continuously withdrawing a grown crys- A. I. ADAMCIK, Assistant Examiner. 

1. APPARATUS FOR RAPIDLY GROWING A MONOCRYSTALLINE WIRE OF SEMICONDUCTIVE MATERIAL FROM A MELT OF THE SAME SAID APPARATUS COMPRISING: 8A) A CRUCIBLE OF INERT REFRACTORY MATERIAL COMPRISING A SOLID BOTTOM SUPPORTING PORTION WITH AN INTEGRAL UPWARDLY EXTENDING SIDE WALL PORTION FORMING AN OPEN TOP RECEPTACLE SUITABLE FOR CONFINING A MELT OF SEMICONDUCTIVE MATERIAL; (B) MEANS FOR INTRODUCING SEMICONDUCTIVE MATERIAL TO BE MELTED INTO SAID CRUCIBLE; (C) MEANS FOR HEATING SEMICONDUCTOR MATERIAL INTRODUCED INTO SAID CRUCIBLE TO A TEMPERATURE AT LEAST AS HIGH AS THE MELTING TEMPERATURE OF THE SEMICONDUCTIVE MATERIAL; (D) A TOP COVER OF INERT REFRACTORY MATERIAL FOR SAID CRUCIBLE HAVING A PERIPHERAL PORTION THEREOF ABUTTING THE SIDE WALL OF SAID CRUCIBLE NEAR THE UPPERMOST EXTREMITY OF SAID SIDE WALL, SAID PERIPHERAL PORTION INCLUDING AN ANNULAR UPPER SURFACE, SAID COVER HAVING AN INVERTED FRUSTOCONICAL-SHAPE CENTRAL PORTION DEPENDING FROM SAID PERIPHERAL PORTION AND HAVING THE INNERMOST PART THEREOF ADAPTED TO BE SUBMERGED UNDER THE SURFACE OF A MELT WITHIN SAID CRUCIBLE, THE CENTERMOST PORTION OF SAID COVER COMPRISING AN ANNULAR LIP PROJECTING UPWARDLY FROM THE INNERMOST PART OF SAID FRUSTOCONICAL-SHAPE CENTRAL PORTION AND ADAPTED TO EXTEND TO AT LEAST THE TOP SURFACE OF A MELT WITHIN SAID CRUCIBLE, AND SAID ANNULAR LIP DEFINING A SMALL DIAMETER CENTRAL APERTURE THROUGH WHICH A CRYSTAL CAN BE WITHDRAWN CONTINUOUSLY FROM A REALTIVELY SMALL CONVEXED PART OF THE TOP SURFACE OF A MELT WITHIN SAID CRUCIBLE; (E) A COOLING SHIELD OF HIGH THERMAL CONDUCTIVITY METAL HAVING A PERIPHERAL PORTION THEREOF ENGAGING SAID ANNULAR UPPER SURFACE OF SAID COVER, SAID SHIELD HAVING A CONTINUOUS PASSAGE THEREIN FOR FLOW OF A COOLING MEDIUM, AND SAID SHIELD HAVING AN INVERTED FRUSTOCONICAL-SHAPE CENTRAL PORTION SPACED FROM SAID COVER AND DEPENDING FROM THE PERIPHERAL PORTION OF SAID SHIELD, THE CENTRAL PORTION OF SAID SHIELD HAVING A CYCLINDRICAL APERTURE THEREIN THAT IS CLOSELY SPACED COAXIAL RELATIONSHIP TO THE CENTRAL APERTURE OF SAID COVER, WHEREBY A HIGH THERMAL GRADIENT IS ESTABLISHED ADJACENT THE TOP OF THE ANNULAR LIP OF SAID COVER WITHOUT SUBJECTING THE MAJOR SURFACE OF A MELT WITHIN SAID CRUCIBLE TO DELETERIOUS COOLING; (F) MEANS FOR CONTINUOUSLY WITHDRAWING A GROWN CRYSTAL FROM THE TOP SURFACE OF A MELT WITHIN THE ANNUCLEAR LIP OF SAID COVER THROUGH THE APERTURES IN SAID COVER AND SAID SHIELD. 